The present invention generally relates to automatic gain control circuits, and more particularly to an automatic gain control circuit of an image reading apparatus.
In a conventional image reading apparatus, a light reflected on a subject copy during a scanning of a light emitted from a light source is received by a light receiving part of the conventional image reading apparatus. In the light receiving part, the reflected light from the subject copy is photoelectrically converted repeatedly for each scanning line to supply an image signal. The image signal is amplified with an image signal gain being automatically adjusted by an automatic gain control circuit. FIGS. 1A and 1B respectively show a conventional automatic gain control circuit 10 and a timing chart for explaining an operation of this conventional automatic gain control circuit 10. This automatic gain control circuit 10 generally has a variable gain amplifier 1, a white signal detect/hold circuit 2 and an error amplifier 3. And, there is provided a white reference plate which is placed outside the range of an effective reading of each scanning line for the subject copy by the image reading apparatus. The scanning of a light emitted from a light source is repeatedly made on the white reference plate during the ON state of the light source for each scan. This scan is called a main scan, and this main scan is made in a main scanning direction, or the emitted light is scanned in a direction from left to right or vice versa with respect to the subject copy to be read by the image reading apparatus. The light is also scanned repeatedly in a direction from top to bottom with respect to the subject copy for each scan. This scan is called a sub scan, and this scan is made in a sub scanning direction. The image signal supplied from the light receiving part for each scan is represented, for example, as an image signal wave shown in FIG. 1B, which is given during an effective reading period within one line scanning period. At the beginning portion of the one line scanning period prior to the start of the image signal wave, a waveform of a white signal WHI as shown in FIG. 1B (a) appears. And, the image signal P.sub.0 from the light receiving part is amplified by the variable gain amplifier 1, and the white signal given from the variable gain amplifier 1 is detected by the white signal detect/hold circuit 2, and a holding of the image signal is made. An output signal of the detect/hold circuit 2 is compared with a predetermined reference signal by the error amplifier 3, and with a resulting error signal represented by an error or difference between the reference signal voltage Vo and the output signal voltage Vi of the detect/hold circuit 2, the amplification by the variable gain amplifier 1 is automatically adjusted. Even with varying quantities of the light supplied from the light source, it is possible for the variable gain amplifier 1 to supply a substantially constant level of the image signal P.sub.1. To ensure a longer useful life of the light source, the light source is turned OFF when an image reading is not performed. For this time period for which the light source is turned OFF, no white signal is supplied from the light receiving part and the output signal Vi of the detect/hold circuit 2 becomes zero voltage, while the gain of the variable gain amplifier 1 is at the maximum level. The output signal Vi of the detect/hold circuit 2 inherently shows a distortion, that is also called a sag, as shown with cl of FIG. 1B (c). This distortion causes varying gains of the detect/hold circuit 2, which may often worsen the accuracy of the image signal P.sub.1 outputted from the variable gain amplifier 1. To avoid this, the detect/hold circuit 2 has a holding circuit part showing an extremely great time constant when compared to a time period of one line scanning of the image reading apparatus.
Japanese Published Patent Application No. 52568/1988 discloses a conventional image reading apparatus. This image reading apparatus comprises an original fixation surface having a reference reflector plate, a light source emitting a light that scans an original on the original fixation surface, the reference reflector plate being placed to face the same surface that the original faces, a sensor for sensing the quantities of a light reflected on the original on the original fixation surface and of a light reflected on the reference reflector plate, and a control unit for controlling the quantity of the light emitted from the light source responsive to an output signal of the sensor so as to obtain a substantially constant emission of the light source. This conventional image reading apparatus employs a fluorescent lamp as the light source. The quantity of light emitted from a fluorescent lamp generally is difficult to control in the entire range of reading the original, and this causes difficulty to obtain a stable image signal level. And the provision of a complicated control mechanism for obtaining a continuously stable level of the image signal suitable for practical use will result in a high cost of manufacturing the image reading apparatus.
In the above described automatic gain control circuit 10, the white signal detect/hold circuit 2 has the holding circuit part having a significantly large time constant when compared to the time period of a single line reading. From a condition in which the light source is turned OFF and the output of the detect/hold circuit 2 is equal to zero voltage, the image reading of the subject copy for each scanning line is started with the turning ON of the light source immediately when a reading start set signal ST1 changes from a high level HI to a low level LO, as indicated by al of FIG. 1C (a). Before the reading is started, however, a significant time period t.sub.2 is required for the output signal Vi of the detect/hold circuit 2 until it reaches a stable voltage level Vst starting from zero voltage, as shown in FIG. 1C (b). It actually takes several seconds or several ten seconds for the image signal P.sub.1 from the variable gain amplifier 1 to reach a stable level. Therefore, the image reading apparatus must unnecessarily wait for such a time period t.sub.2 before it starts the reading operation. In addition, an attempt for improving the accuracy of the image signal level outputted from the variable gain amplifier 1 by reducing a distortion in the output signal of the detect/hold circuit 2 will make the waiting time between the turning ON of the light source and the starting of the reading operation longer than that of the above described case.